In one form of the printing process, printing is effected by photopolymer or rubber printing plates mounted on cylinders or flexographic sleeves, the material to be printed being impressed on the inked printing plate. The cylinder or sleeve on which the printing plates are mounted is generally called the plate or printing cylinder. The quality of a printing job depends, in a large measure, on the care in which pre-press preparations are carried out. Plate-mounting, color registration and proofing are effected off the press by means of commercially available mounting-proofing machines designed for this purpose.
The mounting of photopolymer or other printing plates onto the plate cylinder for printing therefrom requires a high degree of accuracy in the alignment thereof. The image must be straight and in register on the cylinder in order to print straight and in register on the work. In the printing of colors or in the super-impression of images, the various colors or images are added sequentially. Accordingly, it is important that in each case the printing plate which is adding the successive color or image be synchronized with the preceding plate or plates so that the colors or images are accurately superimposed. To arrange these plates in the exact predetermined relation to one another requires that their angular and transverse position on of the printing cylinder as well as their location about a circumference of the printing cylinder be accurately performed. In the prior art, this synchronizing has been performed by manually using mechanical apparatuses which are complicated in their implementation and easily subject to inaccuracies from operator error. In the past, a quality of synchronizing of the printing plates has been realized when they were in position in the printing press. This is not only inconvenient and presents difficult working conditions, but also the printing press is out of operation during this time.
One common method to effect the alignment of the plates with respect to the print cylinder involves the drawing of a line around the print cylinder. This line is then aligned by eye with a longitudinal line along the length of the photopolymer or other print plate. This method is relatively accurate but can be extremely time consuming for the operator. This leads to delay between print runs and is costly with respect to the time lost between such runs.
Alternatively, there is commercially available a device to aid in the alignment of photopolymer or printing plates onto the print cylinder. The print cylinder is placed in a fixed relationship to the device and the plate is laid upside down on a clear glass top. By means of a series of mirrors having lines drawn thereon, the plate is aligned relative to the print cylinder. However, this device is also relatively time consuming and the required accuracy is not achieved. There is only a one-to-one relationship between the eye of the operator and the device assisting in the alignment which can lead to errors of up to one millimeter. These errors are unacceptable where accurate printing is required. This device is generally only acceptable for the alignment of printing plates with respect to one another rather than with respect to the print cylinder. These machines, which usually make use of an optical mounting system, make it possible to mount the plates on plate cylinders to effect exact color registration, a procedure essential to the maintenance of both quality and economy in all flexible plate printing operations.
Another drawback of existing types of mounting machines is their limited capacity to handle printing cylinders of different diameters. With machines of the type heretofore known, the capacity of the machine is restricted to a range of printing cylinder diameters extending from about ninety-five percent of the diameter of the proofing cylinder down to about twenty-five or thirty percent thereof, or approximately four to one. Moreover, since in existing structures the proof forces imposed at contact are eccentrically opposed, the structures required to accommodate these magnified forces are too large to permit smaller sizes of printing cylinders to fit the machine.
Accordingly, it would be desirable to produce an apparatus for mounting a printing plate, wherein the apparatus and a method of operation thereof maximize an accuracy and an efficiency of mounting the printing plate.